Washington -LRB- CNN -RRB- -- Colleagues of Sen. Robert Byrd of West Virginia mourned his death as family and friends planned his funeral .

Byrd , the longest-serving member of the U.S. Congress , died Monday at the age of 92 .

Under West Virginia law , the state 's popular two-term Democratic governor , Joe Manchin , has the power to appoint Byrd 's successor . Manchin is expected to name a fellow member of his party to succeed Byrd , who was also a Democrat , thereby keeping a total of 59 Democrats in the Senate .

There are questions , however , regarding exactly how long Byrd 's appointed successor can serve before another election is held .

West Virginia law says that if a Senate vacancy is created within two and a half years of the end of a term , the appointed successor will automatically serve out the remainder of the term . If not , a special election is required .

Byrd 's current term is scheduled to end on January 3 , 2013 . The two and a half year mark will be reached on Saturday , July 3 .

West Virginia law fails , however , to state exactly when a vacancy occurs . Whether the vacancy is considered to have been created at the moment of Byrd 's death , or when the Senate informs state officials of the vacancy , or when Manchin declares the seat vacant will be crucial . West Virginia -- a traditional Democratic stronghold -- has been increasingly competitive for the Republicans . John McCain easily defeated Barack Obama in West Virginia in the 2008 presidential election .

Neither Manchin nor the Democratic-led Senate have made any official declarations yet .

As question swirl around the timing of the next election for Byrd 's seat , numerous political leaders have been issuing statements in remembrance of the nine-term senator :

-- President Barack Obama

`` The people of West Virginia have lost a true champion , the United States Senate has lost a venerable institution , and America has lost a voice of principle and reason with the passing of Robert C. Byrd . Senator Byrd 's story was uniquely American . He was born into wrenching poverty , but educated himself to become an authoritative scholar , respected leader , and unparalleled champion of our Constitution . He scaled the summit of power , but his mind never strayed from the people of his beloved West Virginia . He had the courage to stand firm in his principles , but also the courage to change over time . ''

-- Gov. Joe Manchin , D-West Virginia

`` Sen. Byrd was a fearless fighter for the constitution , his beloved state and its great people . He made a significant mark as a member of Congress in both our state 's and nation 's history . His accomplishments and contributions will define history for eternity . ''

-- Sen. Jay Rockefeller , D-West Virginia

`` It has been my greatest privilege to serve with Robert C. Byrd in the United States Senate . I looked up to him , I fought next to him , and I am deeply saddened that he is gone . He leaves a void that simply can never be filled . But I am lifted by the knowledge of his deep and abiding faith in God , I have joy in the thought of him reunited with his dear -LRB- late wife -RRB- Erma , and I am proud knowing that his moving life story and legacy of service and love for West Virginia will live on .

Senator Byrd came from humble beginnings in the southern coalfields , was raised by hard-working West Virginians , and triumphantly rose to the heights of power in America . But he never forgot where he came from nor who he represented , and he never abused that power for his own gain . ''

-- Senate Majority Leader Harry Reid , D-Nevada

`` By virtue of his endurance , Robert Byrd knew and worked with many of the greats of the United States Senate . Because of his enduring virtue , he will be remembered as one of them . Senator Byrd dedicated every single day of his Senate service to strengthening the institution , state and republic that he loved so dearly . There will never be another like him . ''

-- Senate Minority Leader Mitch McConnell , R-Kentucky

`` Sen. Byrd combined a devotion to the U.S. Constitution with a deep learning of history to defend the interests of his state and the traditions of the Senate . We will remember him for his fighter 's spirit , his abiding faith , and for the many times he recalled the Senate to its purposes . ... We are glad to know that Senator Byrd and his beloved Erma are reunited . We extend our deepest sympathies to the entire Byrd family . ''

-- Sen. Patrick Leahy , D-Vermont

`` No senator came to care more about the Constitution or to be a more effective defender of our constitutional government than the senior senator from West Virginia . He was a senator 's senator . ... I know him as a mentor and a friend . I was honored to stand with him and fight against assaults on the Constitution and against an unnecessary and costly war in Iraq .

He was a self-educated man who learned much throughout his life and had much to teach us all . He was a symbol of West Virginia , he was an outstanding senator , and he was extraordinary American . ''

-- Sen. Jeff Sessions , R-Alabama

`` It is a sad day for all of us . There was no one who loved the institution of the Senate more , and no one who was a better student of it . ''

CNN 's Ted Barrett , Alan Silverleib , and Paul Steinhauser contributed to this report .

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NEW : President Obama : Senate has `` lost a venerable institution ''

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Questions raised about if/when a special election is required

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Harry Reid , Senate Democratic leader : `` There will never be another like him ''

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Void `` can never be filled , '' says Sen. Jay Rockefeller , D-West Virginia